1. Field of the Invention
The present invention pertains to thin film materials for use in integrated circuits and, more particularly, ferroelectric materials for use in integrated memory circuits. More specifically, the thin film ferroelectric materials are layered superlattice materials that exhibit a low degree of imprinting and polarization fatigue after many repetitions of unidirectional voltage pulses.
2. Statement of the Problem
It is well known that thin film ferroelectric materials may be used in a variety of nonvolatile random access memory devices. For example, U.S. Pat. No. 5,600,587 issued to Koike teaches a ferroelectric nonvolatile random access memory using memory cells consisting of a ferroelectric capacitor and a switching transistor. U.S. Pat. No. 5,495,438 issued to Omura teaches a ferroelectric memory that is formed of ferroelectric capacitors connected in parallel.
The capacitors have ferroelectric materials of different coercive field values and, consequently, can use or store multi-value data. U.S. Pat. No. 5,592,409 issued to Nishimura, et al., teaches a nonvolatile memory including a ferroelectric layer that is polarized by the impressed voltage between two gates.
The polarization or memory storage state is read as a high or lovw current flow across the ferroelectric layer, which permits nondestructive readout. U.S. Pat. No. 5,539,279 issued to Takeuchi, et at., teaches a high speed one-transistor-one-capacitor ferroelectric memory that switches between two modes of operation including a dynamic random access memory ("DRAM") mode and a ferroelectric random access memory ("FERAM") mode.
FIG. 1 depicts an ideal polarization hysteresis curve 100 for ferroelectric thin films. Side 102 of curve 100 is produced by measuring the charge on a ferroelectric capacitor while changing the applied field E from a positive value to a negative value. Side 104 of curve 100 is produced by measuring the charge on the ferroelectric capacitor while changing the applied field E from a negative value to a positive value. The points -E.sub.c and E.sub.c are conventionally referred to as the coercive field that is required to bring polarization P to zero, Similarly, the remnant polarization Pr or -Pr is the polarization in the ferroelectric material at a zero field value. The Pr and -Pr values ideally have the same magnitude, the values are most often different in oractice. Thus, polarizastion measured as 2Pr is calculated by adding the absolute values of the actual Pr and -Pr values even though these values may differ in magnitude. The spontaneous polarization values Ps and -Ps are measured by extrapolating a linear distal end of the hysteresis loop, e.g., end 106, to intersect the polarization axis. In an ideal ferroelectric, Ps equals Pr, but these values differ in actual ferroelectrics due to linear dielectric and nonlinear ferroelectric behavior. A large, boxy, substantially rectangular central region 108 shows suitability for use as a memory by its wide separation between curves 102 and 104 with respect to both coercive field and polarization.
Presently available ferroelectric materials depart from the ideal hysteresis shown in FIG. 1. Researchers have investigated materials for use in integrated ferroelectric devices since the 1970's, but these investigations have not yet been commercially successful due to their departures from the ideal hysteresis. For example, U.S. Pat. No. 3,939,292 issued to Rohrer reports early studies of ferroelectric materials for use in ferroelectric memories were performed on Phase III potassium nitrate In practice, potassium nitrate materials have such low polarizabilities and are so badly afflicted by fatigue and imprint that the materials are practically useless in microelectronic memories.
It is difficult to find ferroelectrics that meet certain commercial requirements.
The best materials for integrated ferroelectric devices are switched using a coercive field that can be obtained from conventional integrated circuit operating voltages, i.e., three to five volts. The materials should have a very high polarization, e.g., one exceeding twelve to fifteen .mu.C/cm.sup.2 determined as 2Pr, to permit the construction of memories having sufficient densities. Polarization fatigue should be very low or nonexistent. Furthermore, the ferroelectric material should not imprint; i.e., the hysteresis curve should not shift to favor a positive or negative coercive field.
FIG. 2 depicts a hysteresis curve 100 next to curve 200. Curve 200 shows the effect of fatigue on curve 100. Fatigue reduces the separation between curves 102 and 104 defining central region 108. Central region 108 progressively becomes smaller and smaller with additional fatigue. This change in separation is due to the creation of point charge defects arising in the ferroelectric material as a consequence of polarization switching and the associated screening effect of the defects on the applied field. Thus, fatigue causes the ferroelectric material to wear out over time due to repeated polarization switching.
U.S. Pat. No. 5,519,234 issued to Araujo, et al., teaches that the fatigue problem of curve 200 is substantially overcome by the use of layered superlattice compounds, such as the "layered perovskite-like" materials described in Smolenskii, et al., "Ferroelectrics and Related Materials," Gordon and Breach (1984). The layered superlattice compounds are capable of providing a thin film ferroelectric material wherein the polarization state may be switched up to at least 10.sup.9 times with less than thirty percent fatigue This level of fatigue endurance provides a significant advance in the art because it is at least an order of magnitude better than the fatigue endurance of other ferroelectrics, e.g., lead zirconium titanate ("PZT") or lead lanthanum zirconium titanate ("PLZT")
According to Section 15.3 of the Smolenskii book, the layered perovskite-like materials or layered superlattice compounds are of three general types:
(I) compounds having the formula A.sub.m-1 S.sub.2 M.sub.m O.sub.3m+3, where A=Bi.sup.3-, Ba.sup.2+, Sr.sup.2+, Ca.sup.2-, Pb.sup.2+, K.sup.+, Na.sup.+ and other ions of comparable size; S=Bi.sup.3- ; and M=Ti.sup.4+, Nb.sup.5+, Ta.sup.5+, Mo.sub.6+, W.sup.6+, Fe.sup.3+ and other ions that occupy oxygen octahedra; PA1 (II) compounds having the formula A.sub.m+1 M.sub.m O.sub.3m+1, including compounds such as strontium titanates Sr.sub.2 TiO.sub.4, Sr.sub.3 Ti.sub.2 O.sub.7 and Sr.sub.4 Ti.sub.3 O.sub.10 ; and PA1 (III) compounds having the formula A.sub.m M.sub.m O.sub.3m+2, including compounds such as Sr.sub.2 Nb.sub.2 O.sub.7, La.sub.2 Ti.sub.2 O.sub.7, Sr.sub.5 TiNb.sub.4 O.sub.17, and Sr.sub.6 Ti.sub.2 Nb.sub.4 O.sub.20.
Smolenskii pointed out that the perovskite-like layers may have different thicknesses, depending on the value of m, and that the perovskite AMO.sub.3 is, in principal, the limiting example of any type of layered perovskite-like structure with m=infinity. Smolenskii also noted that if the layer with minimum thickness (m=1) is denoted by P and the bismuthoxygen layer is denoted by B, then the type I compounds may be described as . . . BP.sub.m BP.sub.m . . . Further, Smolenskii noted that if m is a fractional number, then the lattice contains perovskite-like layers of various thicknesses, and that all the known type I compounds are ferroelectrics.
According to the invention, the layered superiattice materials may be summarized more generally under the formula:
A1.sub.w1.sup.+a1 A2.sub.w2.sup.+a2 . . . Aj.sub.wj.sup.+aj S1.sub.x1.sup.+s1 S2.sub.x2.sup.+s2 . . . Sk.sub.xk.sup.+sk+ B1.sub.y1.sup.+b1 B2.sub.y2.sup.+b2 . . . Bl.sub.yl.sup.+bl Q.sub.z.sup.-2, (1)
where A1, A2 . . . Aj represent Asite elements in the perovskite-like structure, which may be elements such as strontium, calcium, barium, bismuth, lead, and others;
S1, S2 . . . Sk represent superlattice generator ("S-site") elements, which usually is bismuth, but can also be materials such as yttrium, scandium, lanthanum, antimony, chromium, thallium, and other elements with a valence of +3; B1, B2 . . . Bl represent B-site elements in the perovskite-like structure, which may be elements such as titanium, tantalum, hafnium, tungsten, niobium, zirconium, and other elements; and Q represents an anion, which generally is oxygen but may also be other elements, such as fluorine, chlorine and hybrids of these elements, such as the oxyfluorides, the oxychlorides, etc. The superscripts in formula (1) indicate the valences of the respective elements, and the subscripts indicate the number of moles of the material in a mole of the compound, or in terms of the unit cell, the number of atoms of the element, on the average, in the unit cell. The subscripts can be integer or fractional. That is, formula (1) includes the cases where the unit cell may vary throughout the material, e.g. in Sr.sub.0.75 Ba.sub.0.25 Bi.sub.2 Ta.sub.2 O.sub.9, on the average, 75% of the A-sites are occupied by strontium atoms and 25 % of the A-sites are occupied by barium atoms. If there is only one A-site element in the compound, then it is represented by the "A1" element and w2 . . . wj all equal zero. If there is only one B-site element in the compound, then it is represented by the "B1" element, and y2 . . . yl all equal zero, and similarly for the superlattice generator elements. The usual case is that there is one A-site element, one superlattice generator element, and one or two B-site elements, as in the present invention, although formula (1) is written in the more general form to include layered superlattice compounds in which the A and B sites and the superlattice generator site can have multiple elements.
The value of z is found from the equation. EQU (a1w1+a2w2 . . . +ajwj)+(s1.times.1+s2.times.2 . . . +skxk)+(b1y1+b2y2 . . . +blyl)=2z. (2)
Formula (1) includes all three of the Smolenskii type compounds.
The layered superlattice materials do not include every material that can be fit into the formula (1), but only those which spontaneously form themselves into crystalline structures with distinct alternating layers during crystallization. This spontaneous crystallization is typically assisted by thermally treating or annealing the mixture of ingredients. The enhanced temperature facilitates ordering of the superiattice-forming moieties into thermodynamically favored structures, such as perovskite-like octahedra. The term "superlattice generator elements" as applied to S1, S2 . . . Sk, refers to the fact that these metals are particularly stable in the form of a concentrated metal oxide layer interposed between two perovskite-like layers, as opposed to a uniform random distribution of superlattice generator metals throughout the mixed layered superlattice material. In particular, bismuth has an ionic radius that permits it to function as either an A-site material or a superlattice generator; but bismuth, if present in amounts less than a threshold stoichiometric proportion, will spontaneously concentrate as a non-perovskite-like bismuth oxide layer.
Despite the tremendous improvements in low fatigue ferroelectrics attributable to layered superlattice compounds, there remains an imprint problem that is typified by curve 202 of FIG. 2. Curve 202 shows that curve 100 can be imprinted or shifted to the right or lert This imprinting occurs when the ferroelectric material is subjected to repetitive unidirectional voltage pulses. The ferroelectric material retains a residual polarization or bias that shifts sides 102 and 104 in a positive or negative direction with respect to the applied field. Thus, curve 202 has been shifted in a positive direction 204 by repeated negative pulsing of a ferroelectric capacitor. A shift in the opposite direction could also occur due to repetitive pulsing by positive voltage. This type of pulsing represents what happens to the ferroelectric materials as a consequence of repeated unidirectional voltage cycling, such as the sense operations in FERAMs (ferroelectric random access memories). Imprint can be so severe that the ferroelectric material can no longer retain a binary polarization state corresponding to a logical 1 or 0 value.
U.S. Pat. No. 5,592,410 issued to Verhaeghe refers to ferroelectric imprint phenomenon as `compensation.` The '410 patent teaches that the imprint problem can be reversed by pulsing voltage during the write cycle to return the hysteresis loop towards the unimprinted position of FIG. 100, as compared to curve 202. Thus, the imprint problem is reversed by special write operations in which the pulsed voltage is opposite the switching voltage. Stili, the recommended voltage pulsing does not address the entire problem because the imprint phenomenon is a partially irreversible one. The observed imprinting reflects corresponding changes in microstructure of the ferroelectric crystal, e.g., the creation of point charge defects with associated trapping of polarized crystal domains. These changes in microstructure are not all reversible.
There remains a need for ferroelectric thin film materials that are substantially free of the imprint and polarization-fatigue problems.
3Solution to the Problem
The present invention alleviates the problems that are mentioned in the discussion above by providing a ferroelectric thin film which remains essentially free of imprint when it is used under standard integrated circuit operating conditions, i.e., at voltages ranging from .+-. three to five volts or less and temperatures ranging from -55.degree. C. to 150.degree. C. The ferroelectric thin film is useful in integrated circuit memories and provides exceptionally high polarization with boxy hysteresis characteristics. Thin film ferroelectric materials according to the invention show percentage imprint values in the range of only about five to ten percent after 10.sup.10 unidirectional voltage pulses at a temperature of 75.degree. C., and 10.sup.9 pulses at 125.degree. C. Also, their polarizability after voltage cycling remains at a high level, corresponding to greater than 12 .mu.C/cm.sup.2.
Thus, electronic devices containing thin film ferroelectric materials according to the present invention are essentially imprint-free and fatigue-free, This improvement derives from the use of thin film ferroelectric material comprising layered superlattice materials containing an excess of B-site elements In the example below, the layered superlattice material comprised strontium bismuth tantalum niobate made from precursors containing amounts of tantalum and niobium in excess of the stoichiometric amounts. The balanced stoichiometric formula for strontium bismuth tantalum nlobate is: EQU SrBi.sub.2 (Ta.sub.1-x Nb.sub.x).sub.2 O.sub.9, (3)
wherein 0.ltoreq..times..ltoreq.1. The "non-stoichiometric" formula for strontium bismuth tantalum niobate can be written as: EQU (SrBi.sub.2 (Ta.sub.1-x Nb.sub.x).sub.2 O.sub.9).sub.p (Bi.sub.2 O.sub.3).sub.q (Ta.sub.2 O.sub.5).sub.r (Nb.sub.2 O.sub.5).sub.s (SrO).sub.t, (4)
which can be viewed conceptually as a mixture of bismuth-layered superlattice oxide compound and simple oxides of each element. The experimental results show generally that when the thin film is made from a precursor solution in which t=0.0.ltoreq..times..ltoreq.1.0.ltoreq.q.ltoreq.p, Osxs1, Osqsp, and the sum of r plus s is greater than zero and less than p, then good polarizability and imprint characteristics are achieved.
Formula (3) corresponds to the general formula (1) wherein the A-site metal is strontium, the S-site metal (i.e., superlaftice generator) is bismuth, the B-site metals are niobium and tantalum, and z=9. Formula (3) corresponds more specifically to the Smolenskii formula of type 1, where the A-site metal is strontium, the S-site metal is bismuth, the M-site metals are niobium and tantalum, and m 2. Formula (4) corresponds to formula (3), except that it provides for additional, nonstoichiometric amounts of A-, S- and B-site elements.
The thin film ferroelectric material of the invention is preferably less than about 6000 .ANG. thick, and is more preferably less than about 4000 .ANG. thick, with the most preferred thickness being about 2000 .ANG..
Thin films of strontium bismuth tantalum niobate exhibit superior resistance against imprint in the intended environment of use within integrated circuits. For example, preferred devices at 75.degree. C. can withstand 10.sup.10 unidirectional (negative) voltage pulse cycles each having a voltage amplitude of three volts with as little as six percent opposite-state imprint. Similarly, the preferred devices at 125.degree. C. can withstand over 10.sup.9 unidirectional voltage pulse cycles each having a magnitude ranging from three to five volts with less than five percent imprint.
It is, therefore, an object of the invention to provide a precursor containing metal moieties in effective amounts for forming a ferroelectric layered superlattice compound, whereby the precursor contains a relative amount of at least one B-site element greater than the stoichiometrically balanced amount of the at least one B-site element.
A feature of the invention is that the precursor contains a relative amount of at least one A-site element less than the stoichlometrically balanced amount of the at least one A-site element.
Another object of the invention is to provide a precursor containing metal moieties in amounts corresponding approximately to the stoichiometrically unbalanced formula A.sub.a S.sub.b B.sub.c O.sub.[g+(a-1)+(b-2)(1.5)+(c-2)(2.5)], where A represents at least one A-site element, S represents at least one superlattice generator element, B represents at least one B-site element, a.ltoreq.1, b.gtoreq.2, and c&gt;2.4.
Another object of the invention is to provide a precursor in which the metal moieties are strontium (Sr), bismuth (Bi), tantalum (Ta) and niobium (Nb) present in relative amounts corresponding approximately to the stoichiometrically unbalanced chemical formula Sr.sub.a Bi.sub.b (Ta.sub.c Nb.sub.d)O.sub.[g+(a-1)+(b-2)(1.5)+(c+d-2)(2.5)], where a .ltoreq.1, b.gtoreq.2, and (c+d)&gt;2. In one preferred embodiment of the invention, a=1, 2.1.ltoreq.b .ltoreq.2.2, and (c+d)&gt;2, and more preferably 2&lt;(c+d).ltoreq.2.4. This preferred embodiment is particularly effective when (c+d) is approximately 2.3, and proven effective when the ratio c/d is approximately 0.6/0.4.
A further object of the invention is to provide a method for forming a first electrode on a substrate, applying the precursor described above to form a thin film containing the ferroelectric layered superlattice compound, and forming a second electrode on the thin film.
A further object of the invention is to provide a ferroelectric device in an integrated circuit comprising a thin film of layered superlattice material containing a relative amount of at least one B-site element greater than the stoichiometrically balanced amount of the at least one B-site element. In a preferred embodiment of the ferroelectric device of the invention, the layered superlattice material contains a relative amount of at least one A-site element less than the stoichiometrically balanced amount of the at least one A-site element. A feature of the invention is that the thin film contains metal moieties in amounts corresponding approximately to the stoichiometrically unbalanced formula A.sub.a S.sub.b B.sub.c O.sub.[g+(a-1)+(b-2)(1.5)+(c-2)(2.5)], where A represents at least one A-site element, S represents at least one superlattice generator element, B represents at least one B-site element, a.ltoreq.1, b.gtoreq.2, and c&gt;2.
In a preferred embodiment of the ferroelectric device, the thin film contains strontium, bismuth, tantalum and niobium in amounts corresponding approximately to the stoichiometrically unbalanced chemical formula Sr.sub.a Bi.sub.b (Ta.sub.c Nb.sub.d)O.sub.[g+(a-1)+(b-2)(1.5)+(c+d-2)(2.5)], where a.ltoreq.1, b.gtoreq.2, and (c+d)&gt;2. Preferably, a.ltoreq.1, 2.1.ltoreq.b.ltoreq.2.2, and (c+d)&gt;2, and more preferably, 2&lt;(c+d).ltoreq.2.4. This preferred embodiment is particularly effective when (c+d) is approximately 2.3, and proven effective when the ratio c/d is approximately 0.6/0.4.
Another object of the invention is a ferroelectric device comprising a first electrode, a second electrode, and a thin film of layered superlattice material as described above, whereby the thin film is located substantially between the first and second electrodes.